1. Field of the Invention
The present invention relates to a secondary battery and a method of manufacturing the same and, more particularly, to a secondary battery which can be made thin without degrading its performance and a method of manufacturing the same.
2. Description of the Related Art
Most conventional thin batteries are primary batteries. Typical conventional thin secondary batteries are a thin sealed lead battery and a button type nickel-cadmium battery, which have been recently put on market.
Referring to FIG. 1 (partially cutaway perspective view) and FIG. 2 (horizontal sectional view), a conventional thin secondary battery has positive and negative electrodes 2 and 3 each generally having a flat plate shape. Positive and negative electrode plates 2 and 3 are arranged in battery case 1 such that their major surfaces oppose each other. Separator 4 is interposed between electrodes 2 and 3. Electrolyte 5 is filled in battery case 1. Relief valve 6, positive terminal 7 and negative terminal 8 are provided on battery case 1.
As described above, the conventional thin secondary battery has a structure wherein positive electrode plate 2, negative electrode plate 3, and separator 4 which are the main constituting elements of the battery are positioned one above the other in a direction of thickness thereof.
In order to decrease the height of the battery having the above structure, the thicknesses of positive and negative electrode plates 2 and 3 may be reduced. However, the decrease in thickness is limited due to the following reasons.
The service life of the conventional secondary battery having the above structure greatly depends on the thickness of the electrode plate (positive electrode plate). As is well known in the art, when the thickness of the electrode plate is reduced, the service life of the battery is shortened. Taking a conventional lead battery having the above structure as an example, the relationship between the thickness of the electrode plate and the service life under use with a trickle charge is shown in FIG. 3. As is apparent from FIG. 3, the service life of the conventional battery is abruptly shortened when the thickness of the electrode plate is reduced. When the thickness of the electrode plate is 1 mm or less, the battery cannot be repeatedly used as a secondary battery due to the following reason. In the conventional secondary battery structure, the battery reaction sites generated by charge/discharge extends in a direction perpendicular to the major surface of the electrode plate (i.e., the direction of thickness), as indicated by arrows in FIG. 2. In order to cause any battery to function as a secondary battery, a portion which is not associated with the battery reaction, i.e., an energy concentration portion must always be present in the electrode. When the thickness of the electrode plate is reduced, the energy concentration portion disappears upon charge/discharge. Therefore, the secondary battery cannot serve as a battery. This situation also occurs when the battery is cyclically used.
Due to the above reasons, a minimum overall thickness or height of the conventional thin sealed lead battery is 4 to 5 mm.
In order to manufacture the conventional secondary battery, casting, cutting, and rolling of the electrode plates are required, and an active material must be applied to the surface of the electrode plate. Therefore, the manufacturing process is considerably complicated. In addition, according to the conventional method, in order to manufacture batteries having different electrode plate shapes and different battery voltages, different manufacturing lines and individual manufacturing apparatuses are required for manufacturing different batteries. For this reason, it is difficult to satisfy a variety of needs for many types of batteries.